Certain vehicles are propelled at least part of the time using electrical energy drawn from a high-voltage DC battery pack. The battery pack energizes one or more polyphase electric traction motors via a power inverter. Hybrid electric vehicles selectively use an internal combustion engine as a source of input torque to a transmission, alone or in conjunction with the traction motor(s), while extended-range electric vehicles use a smaller engine only when needed, and solely to power an electric generator. Battery electric vehicles forego use of the small gas engine, and instead operate using stored electrical energy or regenerative braking energy. All three vehicle configurations can operate solely on electricity in what is referred to as an electric vehicle (EV) mode.
In all of the above vehicle embodiments, the high-voltage DC battery pack is used to alternatively store and deliver the substantial amounts of electrical energy needed for driving the traction motor(s). The battery pack, which may consist of multiple battery modules each containing multiple cylindrical or flat/tabular battery cells, generates heat in operation. Effectively dissipating the generated heat is essential to optimizing vehicle performance. As a result, thermal systems are used in conjunction with battery packs to circulate a volume of a suitable cooling fluid through the battery pack and any associated power electronics.